Added new setting allow_project_creation_for_guest_and_below

What does this MR do and why?

Adds allow_project_creation_for_guest_and_below which will be used to limit the behaviour of users with highest_role <= Guest.

If the flag is set to true (default), the behaviour would be as current prod.

If the flag is set to false , Guest and lower role would not be allowed to create personal projects, as creating projects can promote themselves into a paid seat.

This MR just adds the application setting.

The next MR adds the logic to control the behaviour

ref: #426279 (closed)

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Before	After

DB steps

ci: == [advisory_lock_connection] object_id: 348860, pg_backend_pid: 97893
main: == [advisory_lock_connection] object_id: 355180, pg_backend_pid: 97896
main: == 20231019093412 AddAllowProjectCreationForGuestAndBelowToApplicationSettings: migrating
main: -- add_column(:application_settings, :allow_project_creation_for_guest_and_below, :boolean, {:default=>true, :null=>false})
main:    -> 0.0014s
main: == 20231019093412 AddAllowProjectCreationForGuestAndBelowToApplicationSettings: migrated (0.0039s)

main: == [advisory_lock_connection] object_id: 355180, pg_backend_pid: 97896
ci: == [advisory_lock_connection] object_id: 361520, pg_backend_pid: 97898
ci: == 20231019093412 AddAllowProjectCreationForGuestAndBelowToApplicationSettings: migrating
ci: -- add_column(:application_settings, :allow_project_creation_for_guest_and_below, :boolean, {:default=>true, :null=>false})
ci:    -> 0.0014s
ci: == 20231019093412 AddAllowProjectCreationForGuestAndBelowToApplicationSettings: migrated (0.0070s)

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